The first objective of this study was to determine the respective and possibly differential contribution of peer rejection and friends’ aggression to reactive aggression and proactive aggression when only one child per family is involved. This analysis corresponds to classical associative studies based on singular results (i.e. the results of one child per family). The second objective was to revisit the previous links while controlling for possible gene-environment correlations through the intra-pair difference method in monozygotic (MZ) twins. This method allows to control for genetic and environmental influences shared by MZ twins living together. The data was collected from 156 pairs of 10-year-old MZ twins (80 pairs of girls); these are part of the Quebec Newborn Twins Study (QNTS). The results show that proactive aggression is related to friends' aggression both in terms of singular outcomes and intra-pair differences. However, this study did not provide convincing results regarding the link between reactive aggression and peer rejection or friends’ aggression. Additional explanations are proposed to understand the results and guide future studies.